1. Field of the Invention
The present invention relates to an M-shaped antenna apparatus, and in particular, to an M-shaped antenna apparatus provided with at least two M-shaped antennas.
2. Description of the Related Art
FIG. 24 is a perspective view showing a construction of a prior art antenna apparatus capable of operating at a plurality of frequencies, and FIG. 25 is an enlarged plan view showing a detailed construction of an antenna element 113 and its peripheries of FIG. 24.
Referring to FIG. 24, the prior art antenna apparatus has a rectangular equipment body which is constituted by a grounding conductor 111 provided on a bottom surface located on an X-Y plane, three rectangular top surface conductors 115a, 115b and 115c provided on a top surface and four side surface conductors 114. On the top surface thereof, a rectangular aperture 116 is formed between the top surface conductor 115a located in an approximate center portion and the top surface conductor 115b, and a rectangular aperture 117 is formed between the top surface conductor 115a and the top surface conductor 115c. In this case, a circular feeding point 118 is provided in an approximate center portion of the top surface conductor 115a. On the other hand, a feeding portion 112 is provided on the grounding conductor 111 just below the feeding point 118, and a center conductor of the feeding portion 112 is connected to the lower end of the antenna element 113. The antenna element 113 is extended in the vertical direction, and its upper end is located at the feeding point 118.
Referring to FIG. 25, at the circular feeding point 118, a gap 120 is formed between the top surface conductor 115a and the upper end of the antenna element 113, and a frequency selection circuit 119 is connected between them. In this prior art antenna apparatus, the grounding conductor 111, the top surface conductors 115a, 115b and 115c and the four side surface conductors 114 are electrically connected to each other, forming a rectangular parallelepiped symmetrically with respect to a Z-Y plane and a Z-X plane. On the top surface, two rectangular apertures 116 and 117 of the same shape are arranged symmetrically with respect to the Z-Y plane, the feeding portion 112 is arranged at the origin of the X-Y plane, and the antenna element 113 is constructed of a conductor line perpendicular to the X-Y plane.
The operation of the antenna apparatus shown in FIGS. 24 and 25 will be described next in detail. According to this antenna apparatus, an antenna formed when the gap 120 is short-circuited by replacing the frequency selection circuit 119 with a conductor is referred to as a first antenna element, and the resonance frequency of the first antenna is denoted by f1. Moreover, an antenna formed when the gap 120 is opened by removing the frequency selection circuit 119 is referred to as a second antenna element, and the resonance frequency of the antenna is denoted by f2. Therefore, the first antenna has a structure in which the antenna element 113 and the top surface conductor 115a are short-circuited to each other, while the second antenna has a structure in which an electric capacity provided by the gap 120 is connected in series between the antenna element 113 and the top surface conductor 115a. With this arrangement, the first and second antennas have different resonance frequencies.
The frequency selection circuit 119 has such a characteristic that it has low impedance at the frequency f1 and high impedance at the frequency f2. If the antenna element 113 and the top surface conductor 115a are connected to each other by means of the frequency selection circuit 119, then the frequency selection circuit 119 is put into a low-impedance state, i.e., almost short-circuited at the frequency f1, and the antenna operates as the first antenna. The circuit is put into a high-impedance state, i.e., almost opened at the frequency f2, and the antenna operates as the second antenna. As described above, this antenna apparatus becomes an antenna apparatus that operates at the two frequencies of the first and second antennas with one antenna structure.
FIG. 26 is a perspective view showing a construction of one implemental example (prototype) of the antenna apparatus of FIG. 24. In this implemental example, a relation between the frequency f1 and the frequency f2 is expressed by the following equation (1).
f2=2.6xc3x97f1xe2x80x83xe2x80x83(1) 
In this case, the free space wavelength of the frequency f1 is denoted by xcex1, and the free space wavelength of the frequency f2 is denoted by xcex2. In this case, the grounding conductor 111 has a rectangular shape constructed of two sides that have a length of 0.72xc3x97xcex1 and a length of 0.56xc3x97xcex1, and the side surface conductors 114 have a height of 0.06xc3x97xcex1. The top surface conductor 115a located in the approximate center portion has a rectangular shape of which the side parallel to the X-axis has a length of 0.26xc3x97xcex1 and the side parallel to the Y-axis has a length of 0.56xc3x97xcex1. The top surface conductors 115b and 115c located at both ends have a rectangular shape of which the side parallel to the X-axis has a length of 0.08xc3x97xcex1 and the side parallel to the Y-axis has a length of 0.56xc3x97xcex1. The two rectangular apertures are the rectangles of which the side parallel to the X-axis has a length of 0.15xc3x97xcex1 and the side parallel to the Y-axis has a length of 56xc3x97xcex1. The electric characteristics of this antenna apparatus when the antenna apparatus has a structure symmetrical with respect to the Z-X plane and the Z-Y plane are as follows.
Further, the antenna element 113 is a conductor line that has a diameter of 0.015xc3x97xcex1 and an element length of 0.06xc3x97xcex1. The frequency selection circuit 119 is constructed of an LC parallel circuit, whose resonance frequency is the frequency f2. As shown in the Smith chart of FIG. 30, this frequency selection circuit 119 becomes a low impedance at the frequency f1 and becomes a high impedance at the frequency f2. Citing one example in which the frequency f2 is 2.14 GHz, a combination of the inductance L and the electrostatic capacity C of the LC parallel circuit is provided as one example in which L=11 nH and C=0.5 pF.
FIG. 27A is a graph showing a voltage standing wave ratio (VSWR) characteristic with respect to a normalized frequency f/f1 of the first antenna element when the frequency selection circuit 119 is replaced by a short-circuit conductor in the antenna apparatus of the implemental example of FIG. 26. FIG. 27B is a graph showing a voltage standing wave ratio (VSWR) characteristic with respect to a normalized frequency f/f2 of the second antenna element when the frequency selection circuit 119 is put in an open state in the antenna apparatus of the implemental example of FIG. 26. FIG. 27C is a graph showing a voltage standing wave ratio (VSWR) characteristic with respect to the frequency of the antenna apparatus provided with the frequency selection circuit 119 in the antenna apparatus of the implemental example of FIG. 26. In this case, the characteristic impedance of the feeding cable connected to the feeding portion 112 of the antenna apparatus is assumed to be 50 xcexa9.
FIG. 27A shows an impedance characteristic of the first antenna in which the frequency selection circuit 119 is replaced by a conductor, and it can be understood that resonance occurs at the center frequency f1. FIG. 27B shows an impedance characteristic of the second antenna from which the frequency selection circuit 119 is removed, and it can be understood that resonance occurs at the center frequency f2. In either one of the antennas, the frequency band whose VSWR is equal to or smaller than two occupies 10% or more in a band width ratio, and a satisfactory characteristic of small loss throughout a wide band is exhibited. FIG. 23C shows an impedance characteristic of a prior art experimental antenna provided with the frequency selection circuit 119, and it can be understood that resonance occurs at the two frequencies f1 and f2. As described above, this antenna apparatus can be provided as an antenna apparatus that has a satisfactory impedance characteristic with little reflection loss at the two frequencies f1 and f2.
Even in this experimental antenna apparatus, the height of the antenna element 113 is 0.06xc3x97xcex1 (=0.16xc3x97xcex2), which is lower than that of the ordinary quarter-wavelength antenna element. This means a capacitive coupling, which occurs between the top surface conductors 115a, 115b and 115c and the grounding conductor 111 of the antenna apparatus and is equivalent to a capacitive load provided at the upper end of the antenna element 113, and this leads to reduction in height of the antenna apparatus.
FIG. 28A is a graph showing a directivity characteristic on the horizontal plane of the frequency f1 in the antenna apparatus of FIG. 26, while FIG. 28B is a graph showing a directivity characteristic on the vertical plane of the frequency f1 in the antenna apparatus of FIG. 26. FIG. 29A is a graph showing a directivity characteristic on the horizontal plane of the frequency f2 in the antenna apparatus of FIG. 26, and FIG. 29B is a graph showing a directivity characteristic on the vertical plane of the frequency f2 in the antenna apparatus of FIG. 26. In this case, one division of the scale of the radiation directivity characteristic corresponds to 10 dB, and the unit is xe2x80x9cdBdxe2x80x9d based on the gain of a dipole antenna. As a unit for representing the gain of the antenna apparatus, there is xe2x80x9cdBixe2x80x9d that is a gain for the radiation electric power from a point wave source, and there is a relation of the following equation (2) between gains xe2x80x9cdBdxe2x80x9d and xe2x80x9cdBixe2x80x9d.
1 dBd=2.15 dBixe2x80x83xe2x80x83(2) 
As is apparent from FIGS. 28 and 29, in this antenna apparatus, with regard to the radiation directivity characteristic in the X-Y plane at the frequency f1, the electric wave radiation in the Y-direction is suppressed, and the electric wave radiation in the X-direction is strengthened. However, with regard to the radiation directivity characteristic in the X-Y plane at the frequency f2, intense radiation occurs in six directions although the electric wave radiation in the Y-direction is suppressed. This is ascribed to the fact that the grading lobe occurs since the depth of the antenna apparatus is 1.43xc3x97xcex2 (=0.56xc3x97xcex1). Moreover, at either frequency, this antenna apparatus scarcely radiates electric waves on the bottom surface side of the antenna apparatus (xe2x88x92Z region in a direction downward of the grounding conductor 111) and radiates very strong electric waves in a +Z region in a direction upward of the top surface of the antenna apparatus. In particular, the directivity characteristic is comparatively strong in a direction obliquely sidewise from the antenna apparatus. In other words, by virtue of the front surface conductor 115a and the grounding conductor 111 that peripherally surround the antenna element 113, the radiation power is reduced toward the bottom surface side of the antenna apparatus, i.e., in the xe2x88x92Z direction.
Moreover, in this antenna apparatus, the rectangular apertures 116 and 117 for radiating electric waves are provided on the top surface of the antenna apparatus, and the antenna element 113 that serves as a radiation source is surrounded by the grounding conductor 111 and the top surface conductor 115a. Accordingly, there is little influence on the radiation electric waves due to the antenna arrangement environment in the direction of the side surface and the direction of the bottom surface of the antenna apparatus. In other words, when installing this antenna apparatus in an indoor ceiling or the like, it is possible to embed the antenna apparatus in the indoor ceiling and align the antenna apparatus with the indoor ceiling so that the top surface of the antenna apparatus opposes the radiation space, for installation of the M-shaped antenna apparatus. With this arrangement, there is provided an antenna apparatus that has no projecting object on the ceiling or the like and is aesthetically desirable with less conspicuousness.
As described above, according to the construction of the prior art antenna apparatus that has a thin type structure, there is provided an antenna that is smaller than the object projecting from the ceiling and aesthetically desirable with less conspicuousness when it is impossible to embed the antenna in the indoor ceiling.
In connection with the prior art example and the experimental example described herein, there has been described the antenna apparatus that has the structure symmetrical with respect to the Z-Y plane and the Z-X plane. In this case, there is the effect that the directivity characteristic of the electric waves radiated from the antenna apparatus become symmetrical with respect to the Z-Y plane and the Z-X plane. As described above, according to the prior art antenna apparatus, there can be provided a compact antenna that resonates at two or more frequencies with a simple structure.
However, the prior art antenna apparatus shown in FIG. 24 has had the problems as follows. As described above, the above structure is able to operate at two or more frequencies. However, since all the resonance frequencies are determined by the shape of the antenna apparatus, there has been required an advanced designing technology for the designing of the resonance frequencies. In particular, when a plurality of frequency bandwidths are used by a plurality of applications, there has been required a further advanced designing technology for designing of the antenna. Accordingly, in this case, it has been inevitable to admit that the prior art structure, which has been unable to freely easily select a plurality of resonance frequencies, has been improper.
Accordingly, an essential object of the present invention is to solve the aforementioned problems and provide a compact light-weight antenna apparatus, having a plurality of resonance frequencies with a design simpler than that of the prior art examples and is capable of obtaining a bilateral directivity characteristic.
In order to achieve the aforementioned objective, according to one aspect of the present invention, there is provided an M-shaped antenna apparatus including at least two M-shaped antenna elements, a grounding conductor, and a feeding portion, the at least two M-shaped antenna elements including first and second M-shaped antenna elements respectively having first and second resonance frequencies different from each other. The first M-shaped antenna element includes: a first transmission conductor; a first radiation conductor connected between one end of the first transmission conductor and the grounding conductor; a second radiation conductor connected between a middle portion of the first transmission conductor and the feeding portion; and a third radiation conductor connected between the other end of the first transmission conductor and the grounding conductor. The second M-shaped antenna element includes: a second transmission conductor; a fourth radiation conductor connected between one end of the second transmission conductor and the grounding conductor; a fifth radiation conductor connected between a middle portion of the second transmission conductor and the feeding portion; and a sixth radiation conductor connected between the other end of the second transmission conductor and the grounding conductor.
In the above-mentioned M-shaped antenna apparatus, the fifth radiation conductor preferably shares at least a part of the second radiation conductor.
In the above-mentioned M-shaped antenna apparatus, the fifth radiation conductor preferably shares a part of the first transmission conductor.
The above-mentioned M-shaped antenna apparatus preferably further includes at least one matching conductor, which has one end grounded and adjusts an input impedance of the M-shaped antenna apparatus.
In the above-mentioned M-shaped antenna apparatus, the other end of at least one matching conductor out of the matching conductors is preferably electrically connected to one of the radiation conductor and the transmission conductor.
The above-mentioned M-shaped antenna apparatus preferably further includes at least one directivity characteristic control conductor, which has one end grounded and changes a directivity characteristic of the M-shaped antenna apparatus.
In the above-mentioned M-shaped antenna apparatus, at least one of the first and second transmission conductors preferably further includes an additional conductor section for changing the width thereof.
In the above-mentioned M-shaped antenna apparatus, a space including at least a part of the M-shaped antenna element is preferably filled with a dielectric body so as to oppose the grounding conductor.
In the above-mentioned M-shaped antenna apparatus, the grounding conductor and at least one of the transmission conductors are preferably each formed of a conductor pattern on a dielectric substrate, and at least one of the radiation conductors is preferably formed of a through hole conductor formed in the dielectric substrate.
In the above-mentioned M-shaped antenna apparatus, the at least two M-shaped antenna elements are preferably formed on an identical plane.
In the above-mentioned M-shaped antenna apparatus, the at least two M-shaped antenna elements are preferably formed on planes different from each other.
According to the present invention, there can be easily provided an antenna apparatus, which has two or more resonance frequencies with a simple structure and is capable of obtaining a bilateral directivity characteristic.
According to another aspect of the present invention, there is provided an M-shaped antenna apparatus including at least three M-shaped antenna elements, a grounding conductor, and a feeding portion. At least three M-shaped antenna elements include first, second and third M-shaped antenna elements having first, second and third resonance frequencies, respectively. The first M-shaped antenna element includes: a first transmission conductor; a first radiation conductor connected between one end of the first transmission conductor and the grounding conductor; a second radiation conductor connected between a middle portion of the first transmission conductor and the feeding portion; and a third radiation conductor connected between the other end of the first transmission conductor and the grounding conductor. The second M-shaped antenna element includes: a second transmission conductor; a fourth radiation conductor connected between one end of the second transmission conductor and the grounding conductor; a fifth radiation conductor connected between a middle portion of the second transmission conductor and the feeding portion; and a sixth radiation conductor connected between the other end of the second transmission conductor and the grounding conductor. The third M-shaped antenna element includes: a third transmission conductor; a seventh radiation conductor connected between one end of the third transmission conductor and the grounding conductor; an eighth radiation conductor connected between a middle portion of the third transmission conductor and the feeding portion; and a ninth radiation conductor connected between the other end of the third transmission conductor and the grounding conductor. At least three M-shaped antenna elements are formed on planes different from each other, and at least two of the first, second and third resonance frequencies are different from each other.
In the above-mentioned M-shaped antenna apparatus, at least three M-shaped antenna elements are preferably formed so as to be parallel to each other, and a length of each of the first, second and third radiation conductors, a length of each of the fourth and sixth radiation conductors and a length of each of the seventh and ninth radiation conductors are preferably set so as to be equal to each other. The fifth radiation conductor preferably shares at least a part of the second radiation conductor, and the eighth radiation conductor shares at least a part of the second radiation conductor. The antenna apparatus preferably further comprises: a fourth transmission conductor for connecting a middle portion of the first transmission conductor with a middle portion of the second transmission conductor; and a fifth transmission conductor for connecting a middle portion of the first transmission conductor with a middle portion of the third transmission conductor.
In the above-mentioned M-shaped antenna apparatus, a length of the fourth transmission conductor and a length of the fifth transmission conductor are preferably set so as to be equal to each other, and lengths of the first, second and third transmission conductors are preferably set so as to be equal to each other.
In the above-mentioned M-shaped antenna apparatus, a length of the fourth transmission conductor and a length of the fifth transmission conductor are preferably set so as to be equal to each other, and at least two of lengths of the first, second and third transmission conductors are preferably set so as to be different from each other.
In the above-mentioned M-shaped antenna apparatus, a length of the fourth transmission conductor and a length of the fifth transmission conductor are preferably set so as to be different from each other, and lengths of the first, second and third transmission conductors are preferably set so as to be equal to each other.
In the above-mentioned M-shaped antenna apparatus, the at least three M-shaped antenna elements are preferably formed so as to be parallel to each other, and a length of each of the fourth and sixth radiation conductors and a length of each of the seventh and ninth radiation conductors are preferably set so as to be equal to each other. The fifth radiation conductor preferably shares at least a part of the second radiation conductor, the eighth radiation conductor shares at least a part of the second radiation conductor. The antenna apparatus preferably further includes: a fourth transmission conductor for connecting a middle portion of the second radiation conductor with a middle portion of the second transmission conductor; and a fifth transmission conductor for connecting a middle portion of the second radiation conductor with a middle portion of the third transmission conductor.
In the above-mentioned M-shaped antenna apparatus, a length of the fourth transmission conductor and a length of the fifth transmission conductor are preferably set so as to be equal to each other, and at least two of lengths of the first, second and third transmission conductors are preferably set so as to be different from each other.
In the above-mentioned M-shaped antenna apparatus, at least three M-shaped antenna elements are preferably formed so as to be parallel to each other, and a length of each of the fourth and sixth radiation conductors and a length of each of the seventh and ninth radiation conductors are set so as to be equal to each other. The fifth radiation conductor preferably shares the second radiation conductor and a tenth radiation conductor whose one end is connected to the second radiation conductor, and the eighth radiation conductor preferably shares the second radiation conductor and the tenth radiation conductor. The antenna apparatus preferably further includes: a fourth transmission conductor for connecting the other end of the tenth radiation conductor with a middle portion of the second transmission conductor; and a fifth transmission conductor for connecting the other end of the tenth radiation conductor with a middle portion of the third transmission conductor.
In the above-mentioned M-shaped antenna apparatus, a length of the fourth transmission conductor and a length of the fifth transmission conductor are preferably set so as to be equal to each other, and at least two of lengths of the first, second and third transmission conductors are preferably set so as to be different from each other.
In the above-mentioned M-shaped antenna apparatus, the grounding conductor preferably has a circular shape.
According to the present invention, there can be easily provided an antenna apparatus, which has three or more resonance frequencies with a simple structure and is able to obtain a symmetrical or asymmetrical bilateral directivity characteristic.